1. Field of the Invention
The present invention relates to a pattern forming process for a metal layer serving as the base of a bump, which process constitutes a part of a process for manufacturing a flip chip IC, in which a metal bump formed on the surface of a semiconductor base is surface-joined to an electrode formed on the surface of a printed circuit board.
2. Description of the Related Art
For further progression of the miniaturization of electronic devices, an improvement in parts mounting density is a key factor. In connection with semiconductor ICs, the development of high-density mounting techniques based on flip chips is being actively carried forward as a replacement for the conventional package mounting.
There are several types of flip chip mounting methods, such as an Au stud bump method and a solder ball bump method. In any of these methods, a barrier metal is provided between the electrode pad of the semiconductor IC and the bump material for the purpose of achieving an improvement in closeness of contact, preventing mutual diffusion, etc. In the case of the solder ball bump method, this barrier metal determines the finish configuration of the bump, so that it is also called BLM (ball limiting metal). The most typical structure of the BLM layer in the solder bump method is a three-layered structure consisting of Cr, Cu and Au layers. Of these, the Cr layer, which is the lowest layer, serves as the layer which comes into close contact with the electrode pad; the Cu layer serves as the layer for preventing diffusion of the solder; and the top layer, that is, the Au layer, serves as the layer for preventing oxidation of the Cu. The patterning of the BLM layer might be performed by a wet etching method using a liquid agent. In that case, however, various problems would be involved, including poor operability, environmental problems due to the waste liquid, poor accuracy in machining, etc.
In view of this, use of a lift-off process as the patterning method for the BLM layer is being considered, in which process a photoresist layer is formed and then separated. In this case, the formation of the BLM layer is usually conducted by using a sputtering apparatus, which leads to a problem in that the formation of the BLM layer tends to extend to the side wall surfaces of the background resist pattern, with the result that the resist separation liquid does not penetrate when the lift-off is to be effected, thereby making it difficult to remove the unnecessary portion of the BLM layer. In view of this, it is necessary to control the configuration of the edge surface of the opening of the photoresist such that it has an overhang-like configuration for the purpose of attaining an improvement in separability in the lift-off operation. This control of the resist configuration might be realized by a lithography process. However, that would involve an increase in the number of steps to be taken. It would be ideal if the control of the resist pattern configuration could be effected during the plasma irradiation, which is usually performed in the process prior to the formation of the BLM layer by sputtering.
Conventionally, a plane parallel plate type plasma processing apparatus as shown in FIG. 3 has been used for the purpose of executing the plasma irradiation prior to the formation of the metal layer. The plasma processing apparatus 1 of FIG. 3 includes a plasma processing chamber 2 in which a vacuum is created; a stage 4 (cathode plate) on which a substrate to be processed 3 is placed; and an anode plate 5 that is opposed to the stage 4, which is connected to a high-frequency power source 6 through the intermediation of a coupling capacitor 7.
When performing the patterning of the metal by the lift-off of the photoresist, the background resist pattern is deformed into an overhang-like configuration by thermal transformation and ion irradiation, and a break is formed at an edge of the BLM layer formed thereon (Any portion where the step coverage of the sputtering layer is rather poor is utilized for this purpose). Then, a resist separation liquid is caused to penetrate through this break to remove the unnecessary portion of the BLM layer to thereby complete the patterning.
However, to perform the control of the background resist pattern through this plasma irradiation in a stable manner, selection of the thickness of the resist layer is also an important factor to be taken into consideration. When a resist pattern of a conventional thickness of approximately 1 .mu.m is used, the region which undergoes thermal transformation by the plasma irradiation will not remain in the resist surface layer but be allowed to reach the interface between that and the background. As a result, the resist layer is bonded to the background, thereby making it difficult to remove the resist pattern by the lift-off in the post-process.
This might be avoided by reducing the plasma irradiation amount. However, a reduction in the plasma irradiation amount would result in an in sufficient deformation in the opening edge of the resist pattern, so that the BLM layer formed will be allowed to reach the side wall of the resist pattern, thereby making it completely impossible for the resist separation liquid to penetrate. Thus, the patterning by lift-off will not be completed (See FIG. 1).
Further, in the conventional pre-metal-layer-formation process, no special attention is paid to the increase in the temperature of the wafer during the process, so that the maximum temperature of the wafer surface when the plasma processing is performed under the normal conditions will generally reach 200.degree. C. to 250.degree. C.
When such a processing is conducted on a specimen wafer on which pattern formation has been performed, the opening edge of the resist pattern will be deformed so as to protrude obliquely upward (See FIG. 2B).
It is to be assumed that this is due to the fact that the original molecular structure of the resist layer surface is destroyed by the excessive heat energy applied rapidly thereto, with the result that it undergoes contraction, this surface stress overcoming the force due to thermal expansion with which the resist end portions would expand laterally.
In this case, the overhang of the resist opening is insufficient, so that sputter particles are allowed to reach the pattern side walls to form a BLM layer. Thus, in the post process of lift-off, the penetration of the separation liquid does not proceed, so that the patterning is not completed. Further, the resist portion which has undergone excessive thermal transformation is carbonized and bonded to the background (See FIG. 2D).
It is accordingly an object of the present invention to provide a metal layer pattern forming method which makes it possible for the configuration of the resist layer to be easily controlled in the pre-processing step prior to the step of forming a BLM (ball limiting metal) layer serving as the multiple metal layer in the formation of a ball bump in a flip chip IC or the like and which does not affect the lower layer.